Observations on background in ALFA for β* = 1000 m MD Sune Jakobsen on behave of the ALFA community

XRPV.A7L1.B2 TCP.C6R7.B2 (vertical) TCP.D6R7.B2 (horizontal) XRPV.B7L1.B2 NIELS BOHR INSTITUTE UNIVERSITY OF COPENHAGEN Background study in ALFA (Beam 2) Sune JakobsenObservations on background in ALFA for β* = 1000 m MD 2/5 Rate in ALFA detectors Scrape one jaw of TCPs in IP7 to 2 σ Nominal Move all ALFA Roman Pots to 10 σ Nominal Move all B7L1U to 4 σ Nominal Move all B7L1L to 4 σ Nominal ReferenceStep out 100 µm with D6R7Step out 1000 µm with D6R7Step out 100 µm with C6R7 Step out 1000 µm with C6R7 ALFA Roman Pots Primary collimators Background study in ALFA Background study in ATLAS Conclusions Moving out Roman Pots

NIELS BOHR INSTITUTE UNIVERSITY OF COPENHAGEN Observations on background in ALFA Background again very high on beam 2 for ALFA (beam 1 for TOTEM). Very large background at 4 σ nominal even with primary collimators at 2 σ nominal Huge reduction in background observed when move out both plan of primary collimators (up to factor 40). Repopulation speed depending steep size. Sune JakobsenObservations on background in ALFA for β* = 1000 m MD 3/5 First 100 µm step with C6R7: rate 9800 Hz and increasing by 1850 Hz/minute (19 %) Second 100 µm step with C6R7: rate 4200 Hz and increasing by 425 Hz/minute (10 %) 1000 µm step with C6R7: rate 720 Hz and increasing by 230 Hz/minute (30 %) Background study in ALFA Background study in ATLAS Conclusions

XRPV.A7L1.B2 TCP.C6R7.B2 (vertical) TCP.D6R7.B2 (horizontal) XRPV.B7L1.B2 NIELS BOHR INSTITUTE UNIVERSITY OF COPENHAGEN Background study in ATLAS (Beam 2) Sune JakobsenObservations on background in ALFA for β* = 1000 m MD 4/5 Rate in ALFA detectors Scrape one jaw of TCPs in IP7 to 2 σ Nominal Move all ALFA Roman Pots to 10 σ Nominal Move all B7L1U to 4 σ Nominal Move all B7L1L to 4 σ Nominal ReferenceStep out 100 µm with D6R7Step out 1000 µm with D6R7Step out 100 µm with C6R7 Step out 1000 µm with C6R7 ALFA Roman Pots Primary collimators Moving out Roman Pots Background study in ALFA Background study in ATLAS Conclusions

NIELS BOHR INSTITUTE UNIVERSITY OF COPENHAGEN Conclusions Background again very high on beam 2 for ALFA (beam 1 for TOTEM). Very large background at 4 σ nominal even with primary collimators at 2 σ nominal. Scraping and then retracting the primary collimators effectually reduces the background, but only proven for positions that will not be allowed during data taking. Repopulation speed depending step size. Sune JakobsenObservations on background in ALFA for β* = 1000 m MD 5/5 Further studies needed and preferably a collimator solution which does not imply any movement/interruptions during data taking. Background study in ALFA Background study in ATLAS Conclusions